…path mangling

5460460 Add AbsPathForConfigVal to consolidate datadir prefixing for path args (James O'Beirne)
a1e1305 Clarify help messages for path args to mention datadir prefix (James O'Beirne)

Pull request description:

  Change `-conf`'s help message to indicate that relative path values will be prefixed by the datadir path. This behavior probably merits clarification; it's kind of confusing when attempting to specify a configuration file in the current directory with `-conf=bitcoin.conf`, but instead loading the `bitcoin.conf` file in ~/.bitcoin datadir.

  ### Edit

  This PR has been modified to document all cases where relative path configurations are modified to be under datadir. A small refactoring has also been added which consolidates this normalization.

Tree-SHA512: be4fc0595fbeba33d17af08f59898af45e76a44f00719ea0282403b155ac6755584604fab765250a3aa14ed6991882c4d1ccbe601184362c5ba97c886bdda344

c25321f Add config changes to release notes (Anthony Towns)
5e3cbe0 [tests] Unit tests for -testnet/-regtest in [test]/[regtest] sections (Anthony Towns)
005ad26 ArgsManager: special handling for -regtest and -testnet (Anthony Towns)
608415d [tests] Unit tests for network-specific config entries (Anthony Towns)
68797e2 ArgsManager: Warn when ignoring network-specific config setting (Anthony Towns)
d1fc4d9 ArgsManager: limit some options to only apply on mainnet when in default section (Anthony Towns)
8a9817d [tests] Use regtest section in functional tests configs (Anthony Towns)
30f9407 [tests] Unit tests for config file sections (Anthony Towns)
95eb66d ArgsManager: support config file sections (Anthony Towns)
4d34fcc ArgsManager: drop m_negated_args (Anthony Towns)
3673ca3 ArgsManager: keep command line and config file arguments separate (Anthony Towns)

Pull request description:

  The weekly meeting on [2017-12-07](http://www.erisian.com.au/meetbot/bitcoin-core-dev/2017/bitcoin-core-dev.2017-12-07-19.00.log.html) discussed allowing options to bitcoin to have some sensitivity to what network is in use. @theuni suggested having sections in the config file:

      <cfields> an alternative to that would be sections in a config file. and on the
                cmdline they'd look like namespaces. so, [testnet] port=5. or -testnet::port=5.

  This approach is (more or less) supported by `boost::program_options::detail::config_file_iterator` -- when it sees a `[testnet]` section with `port=5`, it will treat that the same as "testnet.port=5". So `[testnet] port=5` (or `testnet.port=5` without the section header) in bitcoin.conf and `-testnet.port=5` on the command line.

  The other aspect to this question is possibly limiting some options so that there is no possibility of accidental cross-contamination across networks. For example, if you're using a particular wallet.dat on mainnet, you may not want to accidentally use the same wallet on testnet and risk reusing keys.

  I've set this up so that the `-addnode` and `-wallet` options are `NETWORK_ONLY`, so that if you have a bitcoin.conf:

      wallet=/secret/wallet.dat
      upnp=1

  and you run `bitcoind -testnet` or `bitcoind -regtest`, then the `wallet=` setting will be ignored, and should behave as if your bitcoin.conf had specified:

      upnp=1

      [main]
      wallet=/secret/wallet.dat

  For any `NETWORK_ONLY` options, if you're using `-testnet` or `-regtest`, you'll have to add the prefix to any command line options. This was necessary for `multiwallet.py` for instance.

  I've left the "default" options as taking precedence over network specific ones, which might be backwards. So if you have:

      maxmempool=200
      [regtest]
      maxmempool=100

  your maxmempool will still be 200 on regtest. The advantage of doing it this way is that if you have `[regtest] maxmempool=100` in bitcoin.conf, and then say `bitcoind -regtest -maxmempool=200`, the same result is probably in line with what you expect...

  The other thing to note is that I'm using the chain names from `chainparamsbase.cpp` / `ChainNameFromCommandLine`, so the sections are `[main]`, `[test]` and `[regtest]`; not `[mainnet]` or `[testnet]` as might be expected.

  Thoughts? Ping @meshcollider @laanwj @jonasschnelli @morcos

Tree-SHA512: f00b5eb75f006189987e5c15e154a42b66ee251777768c1e185d764279070fcb7c41947d8794092b912a03d985843c82e5189871416995436a6260520fb7a4db